The invention relates to the field of power electronics.
A circuit arrangement such as this is known, for example, from DE-A-196 15 855. This circuit arrangement has a first power converter, which is connected to a DC voltage source having a first intermediate-circuit capacitor voltage and has three load connections, as well as second power converters, which are connected between the load connections and AC voltage-side connections for a load. At least one second intermediate-circuit capacitor voltage or, if not, zero volts, can be added to or subtracted from a voltage at the respective load connection, by means of one of the second power converters. In this case, the second intermediate-circuit capacitor voltage is less than the first intermediate-circuit capacitor voltage. This makes it possible to achieve a voltage graduation, which is finer than that of the first intermediate-circuit capacitor voltage, for a voltage at the AC voltage-side connections or across the load. A predetermined mean voltage is set at the AC voltage-side connections by the first power converter roughly setting a voltage value in the vicinity of the predetermined voltage, and the second power converter adjusting any remaining difference with respect to the predetermined voltage by pulse duration modulation. In this case, the second power converter is clocked at a faster rate than the first. The second power converters are each fed by means of their own converters and a joint supply transformer. DE-A 197 20 787 describes a method which avoids any feedback of power from the second power converters so that the corresponding converters for supplying the second power converters may be pure rectifiers, that is to say they do not need to have any feedback capability and are thus cheaper. However, both cited cases involve complexity for supplying the second power converters.
One object of the invention is to provide a power-electronic circuit arrangement for transmitting real power of the type mentioned initially, which has a simpler design, and to provide a method for operating this circuit arrangement.
An exemplary circuit arrangement according to the invention has a first power converter and at least one second power converter, with the at least one second power converter not having its own power supply means. Thus, in addition to the connections for the respective load connections of the first power converter and for the respective AC voltage-side connection, the at least one second power converter has no further connections or means for transmitting real power or real energy to or from the second DC voltage intermediate circuit.
There is thus no need for the initially mentioned rectifiers for supplying the second power converters, as a result of which the circuit arrangement becomes considerably simpler, cheaper and more reliable.
In one preferred embodiment, the circuit arrangement according to the invention has a regulating device for joint regulation of at least one intermediate-circuit capacitor voltage of the at least one second power converter, and of a common-mode voltage at the AC voltage-side connections of the circuit arrangement, as well as means for driving semiconductor switches in the first power converter and in the at least one second power converter on the basis of output variables from this regulating device.
This makes it possible to keep mean values, over time, of the voltages of the intermediate-circuit capacitances of the at least one second power converter and, possibly, of the first power converter essentially constant, even though the second power converters are interchanging real power with a load and with the first power converter.
In a further preferred embodiment of the invention, a second power converter is in each case connected in series between a load connection of the first power converter and the associated AC voltage-side connection, with these second power converters each having two two-point inverter bridges. Nominal voltages of the intermediate-circuit capacitors in the first power converter and in the second power converters preferably have a ratio of 2:1 or 3:1. A voltage at the AC voltage-side connections can thus be adjusted with a graduation of 5 to 9 steps, as is shown in the following table:
In a further preferred embodiment of the invention, a second power converter is in each case connected in series between a load connection of the first power converter and the associated AC voltage-side connection, with these second power converters each having a two-point inverter bridge and a three-point inverter bridge. Nominal voltages of the intermediate-circuit capacitors in the first power converter and in the second power converters preferably have a ratio of 2:1, 3:1, 4:1 or 5:1. The voltage at the AC voltage-side connections can thus be adjusted with a graduation of 7 to 15 steps, as shown in the following table:
In a further preferred embodiment of the invention, two second power converters are in each case connected in series between a load connection of the first power converter and the associated AC voltage-side connection, with these second power converters each having two two-point inverter bridges. The voltage at the AC voltage-side connections can thus be adjusted with a graduation from 7 up to a maximum of 27 steps. Nominal voltages of the intermediate-circuit capacitors in the first power converter and in the second power converters preferably have a ratio of 4:1:1, 5:1:1, 6:2:1, 7:2:1, 8:3:1 or 9:3:1.
In an exemplary method according to the invention, at least one voltage of at least one intermediate-circuit capacitance of the at least one second power converter and a common-mode voltage of the AC voltage-side connections of the circuit arrangement are regulated by means of joint regulation. In one preferred variant of the invention, the joint regulation also takes account of a voltage of the intermediate circuit in the first power converter, in particular a neutral point voltage in the intermediate circuit. The joint regulation influences the regulated voltages, taking account of a joint optimization criterion.
In one preferred variant of the invention, a weighted sum of the squares of the errors between the regulated voltages and a respective nominal value is minimized.